The present invention relates to an optical head for recording and/or reproducing a signal on and/or from optical disc type record mediums such as the audio disc, video disc and data disc.
Heretofore, various types of optical heads have been proposed. In such optical heads, a light beam emitted from a light source such as a laser light source is focused onto an optical disc as a fine light spot. In order to effect the recording and reproducing accurately, it is required to correct the mutual displacement between the light spot and a track on the optical disc, i.e. a focusing error and a tracking error.
FIG. 1 illustrates an example of a known optical head in which a focusing error signal is detected by utilizing the critical angle of total reflection. A linearly polarized light beam emitted from a semiconductor laser 1 is converted by a collimator lens 2 into a parallel light flux and then is made incident upon a polarization prism 3. A light flux reflected by a polarizing film 4 of the prism 3 is further reflected by a reflection surface 5 and is made incident upon an objective lens 7 via a quarter wavelength plate 6. The light flux is focused onto an optical disc 8 as a fine light spot having a diameter of about 1 .mu.m. The objective lens 7 is supported by an objective lens actuator 9 movable in two orthogonal directions, i.e. the direction of the optical axis of the objective lens and the radial direction of the optical disc 8 perpendicular to the optical axis. To this end, the actuator 9 comprises a magnet, a yoke and a coil. By means of the actuator 9, the objective lens 7 is driven in the two directions to effect the so-called focus servo and radial servo so that the light spot is always focused at a center of the track in the optical disc 8. A light flux reflected by the optical disc 8 is returned to the polarizing film 4. Due to the function of the quarter wavelength plate 6, the polarization direction of the return light flux is made perpendicular to that of the incident light flux to the prism 3 and thus, the reflected light flux is transmitted through the polarizing film 4 and is made incident upon a light detector 11 via a critical angle prism 10.
The critical angle prism 10 comprises a pair of parallel optical surfaces 10a and 10b which are set so that the incident angle of the optical axis of the light flux reflected from the optical disc 8 is made substantially equal to the critical angle of total reflection. The return light flux is reflected three times between these optical surfaces to enhance the sensitivity of detecting the focusing error. Then, the light flux is made incident upon the light detector 11. The light detector 11 comprises four light receiving regions 11a to 11d divided in the radial direction Y as well as the tangential direction X of the optical disc 8. By suitably processing output signals from the regions 11a to 11d, there are obtained focusing and tracking error signals.
In the known optical head shown in FIG. 1, when the objective lens 7 is situated at a focus position with respect to the optical disc 8, the parallel return light flux is made incident upon the critical angle prism 10 as illustrated in FIG. 2A and thus all light rays in the light flux are made incident upon the optical surfaces 10a and 10b substantially at the critical angle. Therefore, all the light rays are totally reflected by the optical surfaces 10a and 10b and are made incident upon the light receiving regions 11a to 11d uniformly. Contrary to this, when the objective lens 7 is out of focus, the converging or diverging light flux is made incident upon the prism 10 as depicted in FIGS. 2B and 2C, respectively. Then, the return light rays are made incident upon the optical surfaces of prism 10 at angles smaller or larger than the critical angle with respect to a plane including the optical axis and being perpendicular to the incident plane. Therefore, a dark and bright pattern is formed on the light detector 11 viewed in the tangential direction X. The dark and bright pattern formed on the light detector 11 are opposite to each other for the converging and diverging light fluxes as illustrated in FIGS. 2B and 2C. Therefore, it is possible to obtain a focusing error signal from (a+b)-(c+d), wherein a, b, c and d are output signals from the light receiving regions 11a, 11b, 11 c and 11d, respectively. A radial error signal, i.e. tracking error signal may be obtained from (a+c)-(b+d) and a data signal may be derived from (a+b+c+d).
In the optical head illustrated in FIG. 1, in order to obtain good optical properties, the following conditions have to be satisfied.
(1) The light beam emitted from the semiconductor laser 1 is an elliptic beam and is linearly polarized in the minor axis direction and in order to increase MTF, the minor axis of the elliptic beam impinging upon the objective lens 7 should be aligned in the tangential direction X. PA1 (2) In order to increase the sensitivity of the focusing error detection, P polarized light should be made incident upon the critical angle prism 10. PA1 (3) In order to avoid the influence of push-pull noise due to pits on the optical disc upon the focusing error signal, the incident plane to the optical surfaces 10a and 10b of the prism 10 should be aligned in the tangential direction X. PA1 (4) The optical surfaces 10a and 10b of the prism 10 should be adjusted accurately with respect to the optical axis of the incident light flux reflected from the optical disc 8 within .+-.10 seconds.
The first three requirements (1) to (3) can be easily satisfied. As for the fourth requirement, in the known optical head, the semiconductor laser 1 is moved in the tangential direction X so as to incline the light flux emanating from the collimator lens 2 as illustrated in FIG. 3. However, in such an optical head, it is extremely difficult to move the light emitting point of the laser only in the tangential direction X and thus, the angle adjustment requires a very long time even for experienced persons. In order to avoid such a drawback, it may be conceived that only the critical angle prism 10 is rotated about an axis perpendicular to the incident plane to the optical surfaces 10a and 10b of prism 10. However, in this case, since the prism 10 has to be arranged separately from the polarization prism 3, it is impossible to precisely keep the desired adjustment under temperature variations.
As shown in FIG. 4, the semiconductor laser 1 comprises a package 1a and LD chip 1b and dimensions of the package 1a have been normalized. That is, the LD chip 1b is precisely positioned with respect to the package 1a within .+-.50 .mu.m in both directions .DELTA.x and .DELTA.y. Therefore, it is preferable that the semiconductor laser 1 be free from adjustment, when considering the adjustment of the whole optical head.